1. Field of the Invention
The present invention relates to apparatus for driving and tensioning the printing ribbon of a printer.
2. Description of the Prior Art
In present day printers, a character carrier is caused to move past a writing support, that is to say a combination of printing paper and inking ribbon. The character carrier makes a cyclic movement such that all the characters which can be printed become available at each printing position. Each printing position has a corresponding actuator which releases or activates a print hammer at the required time, that is to say when the character to be printed is disposed in the appropriate position for printing.
The main types of character carriers are drums of the rotary type or else chain-linked characters or character-bearing belts which move in a straight line.
Given the nature of the duties which they are called upon to perform, it is necessary for printers consistently to provide printing of a high standard and which ensures that the characters printed are perfectly legible. To this end, it is necessary, on the other hand, for a single ribbon to be capable of providing a large number of high grade impressions when passed through the printer a number of times in both directions and on the other hand, for this ribbon to be under constant and adequate tension so that it will always be very accurately positioned relative to the printing paper and the character carrier. In this way, the ribbon is prevented from transferring too much ink to the character carrier and the paper, which are thus kept cleaner so as to minimize maintenance.
In the particular case where the character carrier is an endless type-bearing belt, the upper edge of which is provided with a plurality of character-bearing fingers in the form of flexible tongues, any slackening of the ribbon is likely to cause jamming or foul up of the ribbon in and between the fingers. This could result in damage, on the one hand, to the ribbon itself and, on the other hand, to one or more character-bearing fingers. It is, therefore, important that the arrangement for tensioning the ribbon should be safe and reliable.
Arrangements for tensioning printing ribbons which meet these requirements are known, but generally require large numbers of mechanical parts and are costly.
Whatever the type of arrangement employed, the ribbon is normally tensioned between two support rollers one of which is responsible for winding the ribbon off and the other for winding it on.
In one known arrangement, a single reduction motor (an electric motor associated with a speed reducer) is mechanically coupled to both of the support rollers by a system of gears associated with two braking clutches (preferably of the electromechanical type), the first of which is mechanically coupled to the winding-on support roller and the second to the winding-off support roller. When the first braking clutch is driving, the second braking clutch is disengaged, but brakes the winding-off support roller to which it is coupled and thus, allows the ribbon to be tensioned.
This first arrangement has several known disadvantages. On the one hand, because of the large number of mechanical parts required, it is difficult to manufacture and its cost of manufacture is high. Further, it is always possible for the ribbon to slacken as a result of faults which may occur in the voltage supply to the electromechanical brake resulting in damage to the ribbon or the fingers.
In a second known arrangement, each of the support rollers is mechanically coupled to a reversible reduction motor, with the resisting torque from the reduction units being less than or equal to the braking torque required to tension the ribbon. The motor part of the reduction motor associated with the winding-off roller is supplied with less than its rated supply voltage so that the total resisting torque from the reduction motor enables the ribbon to be suitably tensioned. The motor is used as a brake and its resisting torque is added to the resisting torque from the reduction unit.
This arrangement also has several drawbacks. For example, it is known that in such arrangements the braking torque is not constant because the speed of the winding-off roller varies from -40% to +40% on averge relative to the nominal speed of the motor. Therefore, it is difficult to obtain a relatively constant braking torque. Further, the price of the reduction units of the reduction motors is very high because they have to be extremely accurate.
Both of the aforementioned arrangements for tensioning a printing ribbon incorporates a device for gauging or monitoring the tension of the ribbon so that it is known at all times whether the ribbon is correctly tensioned. This further adds to the cost of the arrangement.